Responsive Landing page

This is my first landing page , I found some difficulties width :

1. Texts sizing - It was difficult to define headers and paragraphs sizes I don't know a specific method to follow , so the media queries code is too long (Redefined all font-sizes).
2. SVG icons - I didn't know how to style the (Social media) DOWNLOADED SVG icons !?? , so I imported them from Fontaesome library.
3. Email validation errors - I didn't find a way to change email validation messages (tooltip and the invalid email message) .
   Please if you can answer one of this questions . I need your help :)